Process for producing polyethylene pipe resins

ABSTRACT

A polyethylene resin comprising from 35 to 49 wt. % of a first polyethylene fraction of high molecular weight and from 51 to 65 wt. % of a second polyethylene fraction of low molecular weight, the first polyethylene having a density of up to 0.930 g/cm 3 , and an HLMI of less than 0.6 g/10 min and the second polyethylene fraction comprising a high density polyethylene having a density of at least 0.969 g/cm 3  and an MI 2  of greater than 10 g/10 min, and the polyethylene resin, having a density of greater than 0.946 g/cm 3 , an HLMI of from 1 to 100 g/10 min, a dynamical viscosity, measured at 0.01 radians/second, greater than 200,000 Pa.s and a ratio of the dynamical viscosities measured at, respectively 0.01 and 1 radians/second greater than 8.

The present invention relates to polyethylene resins, especially tothose suitable for use as pipe resins, and to a process for producingsuch resins. The present invention also relates to the use ofpolyethylene resins for the manufacture of pipes. The present inventionyet further relates to polyethylene pipes.

Polyolefins such as polyethylenes which have high molecular weightgenerally have improved mechanical properties over their lower molecularweight counterparts. However, high molecular weight polyolefins can bedifficult to process and can be costly to produce. Polyolefins having abimodal molecular weight distribution are desirable because they cancombine the advantageous mechanical properties of high molecular weightfraction with the improved processing properties of the low molecularweight fraction.

For many HDPE applications, polyethylene with enhanced toughness,strength and environmental stress cracking resistance (ESCR) isimportant. These enhanced properties are more readily attainable withhigh molecular weight polyethylene. However, as the molecular weight ofthe polymer increases, the processibility of the resin decreases. Byproviding a polymer with a broad or bimodal MWD, the desired propertiesthat are characteristic of high molecular weight resin are retainedwhile processability, particularly extrudability, is improved.

There are several methods for the production of bimodal or broadmolecular weight distribution resins: melt blending, reactor in seriesconfiguration, or single reactor with dual site catalysts. Melt blendingsuffers from the disadvantages brought on by the requirement of completehomogenisation and high cost. Use of a dual site catalyst for theproduction of a bimodal resin in a single reactor is also known.

Metallocene catalysts are known in the production of polyolefins. Forexample, EP-A-0619325 describes a process for preparing polyolefins suchas polyethylenes having a multimodal or at least bimodal molecularweight distribution. In this process, a catalyst system which includesat least two metallocenes is employed. The metallocenes used are, forexample, a bis(cyclopentadienyl) zirconium dichloride and anethylene-bis(indenyl) zirconium dichloride. By using the two differentmetallocene catalysts in the same reactor, a molecular weightdistribution is obtained which is at least bimodal.

EP-A-0881237 discloses the production of bimodal polyolefins withmetallocene catalysts in two reaction zones. The metallocene catalystcomponent comprises a bis-tetrahydro indenyl compound of the generalformula (IndH₄)₂R″MQ₂ in which each IndH₄ is the same or different andis tetrahydroindenyl or substituted tetrahydroindenyl, R″ is a bridgewhich comprises a C₁-C₄ alkylene radical, a dialkyl germanium or siliconor siloxane, or an alkyl phosphine or amine radical, which bridge issubstituted or unsubstituted, M is a Group IV metal or vanadium and eachQ is hydrocarbyl having 1 to 20 carbon atoms or halogen. Thatspecification discloses that the density of the multimodal polyolefinresins particularly falls in the range 0.9 to 0.97 g/ml, preferably 0.92to 0.97 g/ml and that the HLMI of the polyolefin resins particularlyfalls within the range 0.1 to 45,000 g/10 min, preferably in the range0.4 to 45,000 g/10 min. Thus, that specification discloses theproduction of polyolefin resins having a wide variety of properties.

EP-A-0989141 also discloses a process for the preparation ofpolyethylenes having a multimodal molecular weight distribution. Thecatalyst may employ a metallocene catalyst comprising abis-tetrahydroindenyl compound as disclosed in EP-A-0881237. Thespecification discloses the production of pipe resins. Although the piperesins disclosed had good mechanical properties, there is still a needto improve the mechanical properties. There is a need to produce apolyethylene resin having improved mechanical properties yet with goodprocessibility.

Polyethylene resins are known for the production of pipes. Pipe resinsrequire high resistance against slow crack growth as well as resistanceto crack propagation yielding impact toughness.

Pipe resins are known in the art which are referred to by the names “PE80” and “PE 100”. These are polyethylene resins which when formed intopipes of specific dimensions, survive a long term pressure test atdifferent temperatures for a period of 5,000 hours. Extrapolation showsthat they have a 20° C.-50 years resistance of at least 8 and 10 MPa,respectively. This classification is described in ISO 9080 and ISO12162. It is known in the art that the key components for a good PE100resin are the blending of a low molecular weight high densitypolyethylene with little or no short chain branching (SCB) due tocomonomer incorporation and a linear low density polyethylene (LLDPE)resin with high molecular weight and SCB. Known pipe resins have a tradeoff between mechanical properties and processibility. Despite this,there is still a need to improve known pipe resins.

Usually, polyethylene pipe resins in the form of chemical blends areproduced in a cascade reactor process using Ziegler-Natta catalysts.

These known PE100 resins have in general a rheological behaviour thatcould be improved. They generally have a relatively low viscosity at lowshear rates. In particular, the difference between their viscosity atlow shear rate and their viscosity at high shear rate is rather small.This means that during the extrusion of these resins for the manufactureof pipes, sagging can occur. Moreover the injection-moulding capabilityfor the known PE100 resins is not optimal and renders them moredifficult to use for the production of injection moulded pipe fittings.

The present invention aims to overcome the disadvantages of the priorart, in particular by providing improved polyethylene pipe resins.

The present invention provides a polyethylene resin comprising from 35to 49 wt % of a first polyethylene fraction of high molecular weight andfrom 51 to 65 wt % of a second polyethylene fraction of low molecularweight, the first polyethylene fraction comprising a linear low densitypolyethylene having a density of up to 0.930 g/cm³, and an HLMI of lessthan 0.6 g/10 min and the second polyethylene fraction comprising a highdensity polyethylene having a density of at least 0.969 g/cm³ and an MI₂of greater than 10 g/10 min, and the polyethylene resin having a densityof greater than 0.946 g/cm³, an HLMI of from 1 to 100 g/10 min, adynamical viscosity η_(0.01), measured at 0.01 radian/second, greaterthan 200,000 Pa·s and a ratio of the dynamical viscosities measured at,respectively 0.01 and 1 radian/second, η_(0.01)/η₁ greater than 8.

The present invention further provides the use of such a polyethyleneresin for the manufacture of pipes and fittings.

The invention also provides a pipe or a fitting comprising thepolyethylene resin of the invention.

The present invention further provides a process for the preparation ofa polyethylene resin having a bimodal molecular weight distributionwhich comprises:

-   (i) contacting ethylene monomer and a first co-reactant with a    catalyst system in a first reaction zone under first polymerisation    conditions to produce a first polyethylene; and-   (ii) contacting ethylene monomer and a second co-reactant with a    catalyst system in a second reaction zone under second    polymerisation conditions to produce a second polyethylene different    from the first polyethylene;    wherein the first and second polyethylenes are blended together, to    form a polyethylene resin comprising a blend of from 35 to 49 wt %    of a first polyethylene fraction of high molecular weight and from    51 to 65 wt % of a second polyethylene fraction of low molecular    weight, the first polyethylene fraction comprising a linear low    density polyethylene having a density of up to 0.930 g/cm³, and an    HLMI of less than 0.6 g/10 min and the second polyethylene fraction    comprising a high density polyethylene having a density of at least    0.969 g/cm³ and an MI₂ of greater than 10 g/10 min, and the    polyethylene resin having a density of greater than 0.946 g/cm³, an    HLMI of from 1 to 100 g/10 min, a dynamical viscosity η_(0.01),    measured at 0.01 radians/second, greater than 200,000 Pa·s and a    ratio of the dynamical viscosities measured at, respectively 0.01    and 1 radians/second, η_(0.01)/η₁ greater than 8, wherein one of the    co-reactants is hydrogen and the other is a comonomer comprising a    1-olefin containing from 3 to 12 carbon atoms each catalyst system    comprising (a) a metallocene catalyst component comprising a bis    tetrahydroindenyl compound of the general formula (IndH₄)₂R″MQ₂ in    which each IndH₄ is the same or different and is tetrahydroindenyl    or substituted tetrahydroindenyl, R″ is a bridge which comprises a    C₁-C₄ alkylene radical, a dialkyl germanium or silicon or siloxane,    or an alkyl phosphine or amine radical, which bridge is substituted    or unsubstituted, M is a Group IV metal or vanadium and each Q is    hydrocarbyl having 1 to 20 carbon atoms or halogen; and (b) a    cocatalyst which activates the catalyst component.

The polyethylene resins in accordance with the invention have adynamical viscosity η_(0.01), measured at 0.01 radian/second, which isgreater than 200,000 Pa·s. In contrast, known pipe resins produced usingZiegler-Natta catalysts have a η_(0.01) less than 200,000 Pa·s.

In addition, the polyethylene resins in accordance with the inventionhave a η_(0.01)/η₁ ratio greater than 8, preferably greater than 10,where η₁ is the dynamic viscosity at 1 radian/second, expressed in Pa·s.In contrast, known pipe resins produced using a Ziegler-Natta catalysthave a η_(0.01)/η₁ ratio typically much less than 8, most typicallyaround 5.

The determination of dynamical viscosity is made by using an oscillatoryrheometer, preferably a Rheometric Scientific ARES rheometer. Thismethod has been extensively described in the literature devoted topolymer rheology (see e.g. W. W. Graessley, Chapter 3 in PhysicalProperties of Polymers, 2nd Edition, ACS Professional Reference Book,Washington D.C., 1993).

The measurements are performed on a Rheometric Scientific ARES rheometerbetween two 25 mm diameter plates; the gap between the plates is between1 and 2 mm, and is thoroughly adapted according to the suitablethickness of the polymer sample once this latter has been insertedbetween the plates and warmed up to 190° C. The gap value is thenrecorded to be taken into account by the calculation software.

The sample is then temperature-conditioned for a period of 5 minutesbefore the measurement is started. The measurement is performed at 190°C. After temperature conditioning, the measurement starts by applying anoscillatory strain γ*(ω,t)=γ_(M).e^(iωt), with a given amplitude γ_(M)and a given frequency ω to the bottom plate via a precision motor,whereas the top plate is kept fixed. The amplitude γ_(M) of this shearstrain has been chosen in the linear zone of viscoelasticity of thepolymer and is kept constant through the whole experiment. Theoscillation frequency ω is varied through the range [10⁻²-10⁺²]radians/second. The oscillating shear strain is translated inside thematerial into an oscillating shear stress σ*(ω,t), which in-phase andout-of-phase components are recorded as functions of the frequency ω,and used for the calculation of the complex modulus G*(ω) as well ascomplex viscosity η*(ω) of the polymer: $\begin{matrix}{{G^{*}(\omega)} = {\frac{\sigma^{*}\left( {\omega,t} \right)}{\gamma^{*}\left( {\omega,t} \right)} = {{{G_{m}(\omega)} \cdot {\mathbb{e}}^{{\mathbb{i}}\quad{\delta{(\omega)}}}} = {{G^{\prime}(\omega)} + {i \cdot {G^{''}(\omega)}}}}}} \\\begin{matrix}{{{G_{m}(\omega)} = \sqrt{{G^{\prime 2}(\omega)} + {G^{''\quad 2}(\omega)}}};} & {{\tan\quad\delta\quad(\omega)} = \frac{G^{''}(\omega)}{G^{\prime}(\omega)}}\end{matrix} \\{{\eta^{*}(\omega)} = {{{\eta^{\prime}(\omega)} - {i \cdot {\eta^{''}(\omega)}}} = {\frac{G^{''}(\omega)}{\omega} - {i \cdot \frac{G^{\prime}(\omega)}{\omega}}}}} \\{{{\eta^{*}(\omega)}} = \frac{\sqrt{{G^{\prime 2}(\omega)} + {G^{''\quad 2}(\omega)}}}{\omega}}\end{matrix}$

According to the Cox-Merz rule, the function of the absolute value ofthe complex viscosity ∥η*(ω)∥ is the same as the conventional viscosityfunction, (capillary viscosity as a function of shear rate γ), iffrequency is taken in rad/s. If this empiric equation is valid, theabsolute value of the complex modulus corresponds to the shear stress inconventional (that is steady state) viscosity measurements.

In the present invention, the dynamic viscosities of the resin measuredat 0.01 and 1 rad/s respectively according to the aforementioned methodare defined as η_(0.01)=∥η*(0.01 rad/s)∥ and η₁=∥η*(1 rad/s)∥.

The polyethylene resins in accordance with the invention preferablysatisfy the following relationship:η_(0.01)/η₁≧{(0.293×M _(w) /M _(n))+3.594}

The polyethylene resins in accordance with the invention preferablysatisfy the following relationship:η_(0.01)/η₁≧{(−0.302×HLMI)+9.499}

The resin according to the present invention preferably comprises atleast 55% by weight of the second polyethylene fraction of low molecularweight, most preferably at least 56 weight %.

The resin according to the present invention preferably comprises notmore than 45% by weight of the first polyethylene fraction of highmolecular weight, most preferably at most 44 weight %.

In this specification the melt index MI₂ and high load melt index HLMIare measured in accordance with ASTM D-1238 at 190° C. with respectiveloads of 2.16 and 21.6 kg.

Preferably, for the high density polyethylene fraction the MI₂ is from100 to 1000 g/10 min, more preferably from 300 to 1000 g/10 min.

Preferably, for the low density polyethylene fraction, the HLMI is from0.001 to 0.5 g/10 min, more preferably from 0.01 to 0.25 g/10 min.

Preferably, for the polyethylene resin, the HLMI is from 5 to 90 g/10min, more preferably from 10 to 80 g/10 min.

In this specification the density is measured in accordance with ISO1183.

For the low density polyethylene fraction, the density is preferablyfrom 0.908 to 0.927 g/cm³, more preferably from 0.912 to 0.926 g/cm³.

For the high density polyethylene fraction, the density is preferablyfrom 0.970 to 0.990 g/cm³, more preferably from 0.971 to 0.980 g/cm³.

Preferably, the density of the polyethylene resin is from 0.949 to 0.960g/cm³, more preferably from 0.952 to 0.958 g/cm³.

Particularly preferred is a resin having a density of at least 0.954g/cm³.

For the high density polyethylene fraction, the polydispersity index D(represented by the ratio Mw/Mn as determined by gel permeationchromatography (GPC)) is preferably from 2 to 4. For the linear lowdensity polyethylene fraction of high molecular weight the value ofpolydispersity index D is preferably from 3 to 6.

Preferably, the polyethylene resin has a molecular weight distributionMw/Mn from 8 to 30.

Preferably, the low density fraction is a copolymer of ethylene andanother alpha-olefin containing from 3 to 12 carbon atoms. Morepreferably, the low density fraction is a copolymer of ethylene andbutene, methylpentene, hexene and/or octene.

Preferably, the high density fraction is an ethylene homopolymer.

The polyethylene resins in accordance with the invention have in generala lower capillary viscosity μ₂ than commercial PE100 resins. Preferably,μ₂ is less than 21,000 dPa.s. In contrast, known pipe resins producedusing Ziegler-Natta catalysts, have a high capillary viscosity whichrenders them rather difficult to extrude, they typically have a μ₂greater than 21,000 dPa.s. μ₂ is the value of capillary viscosity whichis measured by extruding polymer by means of an extrusion device, whichincorporates a piston in a cylinder, at a temperature of 190° C. througha cylindrical die of length 30 mm and diameter 2 mm at a constant speedcorresponding to a shear rate of 100 s⁻¹ and by measuring the forcetransmitted by the piston during the decent of the piston. The cylinderand piston used by this test method meet the requirements of thecylinder/piston device used for fluidity index measurements according tothe standard ASTM D 1238 (1996). The μ₂ value is then calculated byusing the equation: μ₂=23.61×Fp where Fp represents the mean forceexerted by the piston during the measurement period and is expressed indecaNewtons (daN) whereas μ₂ is express in dPa.s.

The polyethylene resins according to the invention, having such aspecific composition, molecular weight and density, can lead to a markedimprovement of the processing properties when the resin is used as apipe resin, while conserving or improving mechanical behaviour ascompared to known pipe resins.

The polyethylene resins in accordance with the invention can outperformthe current best available bimodal polyethylene resins of PE 100 gradefor properties relating to the fabrication and use of polyethylenepipes.

In particular, the polyethylene resins in accordance with the inventionhave impact resistance and slow crack resistance at least equivalent,often higher than current available PE 100 grade resins.

The resins of the invention are endowed with excellent rheologicalbehaviour, that is they have a similar or lower viscosity at highershear rates (typically around 100 s⁻¹) and a much higher viscosity atlow shear rates (0.1 s⁻¹ or below). These resins provide a reducedsagging following extrusion of the resin into a pipe together with animprovement of the injection moldability.

The resin in accordance with the invention is characterised by a highershear-thinning behaviour which is better than known bimodal PE 100resins. This means less sagging of the polyethylene resins when beingextruded at low shear rates when forming pipes, and goodinjection-moulding capability for the resins when used to produceinjection moulded pipe fittings.

The resins according to the invention permit to obtain pipes having agood resistance to slow crack growth.

In this specification in order to assess the slow crack growthresistance of pipe resins, an environmental stress cracking resistance(ESCR) Bell test in accordance with ASTM-D-1693 at 70° C., with 35%Antarox CO₆₃₀ was employed, and the times to failure were measured. Theresins produced in accordance with the invention have a very high ESCRresistance as measured by the Bell test, the failure times typicallybeing greater than 400 hours, most often even greater than 500 hours.

The slow crack growth resistance of the resins was also tested by a fullnotch creep test (FNCT) according to ISO DIS 16770 in which the time forfailure was recorded for a circumferentially notched specimen having a10 mm×10 mm cross section, the specimen having been submitted to a nettensile strength of 5 MPa at a temperature of 80° C. in a 2% solution ofArkopal N100. Generally, the resins in accordance with the inventionexhibit a time to failure under the FNCT test specified here above of atleast 500 hours, indicating good slow crack growth resistance.

For some of the resins, the slow crack growth resistance was furthertested by a notched pipe test (NPT) in accordance with ISO 13479 under astress of 4.6 MPa at 80° C. using pipes of 110 mm diameter (SDR 11).

Furthermore, the resins in accordance with the invention exhibit goodresistance to rapid crack propagation. In order to assess resistance ofthe resins to rapid crack propagation (RCP), pipes having a diameter of110 mm (SDR11) were subjected to the test according to ISO DIS 13477(the small scale steady state (S4) test) at a pressure of 10 bars todetermine the critical temperature of fracture. A Charpy impact energytest was also carried out at a temperature of −25° C. using theprocedures of ISO 180/1A.

Furthermore, the resins according to the invention have a good creepresistance. The creep resistance was measured according to the test ofISO 1167 on 32 mm diameter SDR 11 pipes to determine the lifetime priorto failure at a temperature of 20° C. and a pressure of 13 MPa.

The pipe resins according to the invention have a creep resistancemeasured according to ISO 1167 of at least 400 hours for a temperatureof 20° C. and a pressure of 13 MPa. The resins produced in accordancewith the invention generally have a creep resistance which is such thatthey can be assigned a minimum required strength (MRS) rating accordingto the ISO/TR 9080 standard which is the MRS 10 rating or even higher,such as a MRS 11.2 rating or even a MRS 12.5 rating. This rating isdetermined according to a statistical method and the minimum requiredstrength MRS is defined as the lower prediction limit (LPL) at a 97.5%confidence interval.

The resins according to the invention are therefore well suited for themanufacture of pipes, in particular high pressure pipes. The presentinvention therefore also concerns the use of the polyethylene resinsaccording to the invention for the manufacture of pipes and fittings andthe pipes and fittings thereby obtained. When used for the manufactureof pipes, the resins are most often blended with usual additives such asanti-oxydants, anti-acids and colorants.

The polyethylene resins according to the invention can be prepared byany method suitable therefore. They can be prepared by physicallyblending the high density and the low density polyethylene fractions,prepared separately, or they can be prepared by polymerising ethylene inthe presence of a mixture of catalysts. Preferably, the high density andlow density fractions are produced in at least two separate reactors,most preferably two such reactors in series. In such a case, the highdensity fraction is preferably prepared first, so that the low densityfraction is prepared in the presence of the high density fraction.

The catalyst employed in the polymerisation process may be anycatalyst(s) suitable for preparing the low and high density fractions.Preferably, the same catalyst produces both the high and low molecularweight fractions in general, a metallocene catalyst is used. Themetallocene catalyst component preferably comprises abis-tetrahydroindenyl compound (THI) of formula (IndH₄)₂R″MQ₂ in whicheach IndH₄ is the same or different and is tetrahydroindenyl orsubstituted tetrahydroindenyl, R″ is a bridge which comprises a C₁-C₄alkylene radical, a dialkyl germanium or silicon or siloxane, or analkyl phosphine or amine radical, which bridge is substituted orunsubstituted, M is a Group IV metal or vanadium and each Q ishydrocarbyl having 1 to 20 carbon atoms or halogen.

The preferred bis tetrahydroindenyl catalyst may be substituted in thesame way or differently from one another at one or more positions in thecyclopentadienyl ring, the cyclohexenyl ring and the ethylene bridge.Each substituent group may be independently chosen from those of formulaXR_(v) in which X is chosen from group IVB, oxygen and nitrogen and eachR is the same or different and chosen from hydrogen or hydrocarbyl offrom 1 to 20 carbon atoms and v+1 is the valence of X. X is preferablyC. If the cyclopentadienyl ring is substituted, its substituent groupsmust not be so bulky as to affect coordination of the olefin monomer tothe metal M. Substituents on the cyclopentadienyl ring preferably have Ras hydrogen or CH3. More preferably, at least one and most preferablyboth cyclopentadienyl rings are unsubstituted.

In a particularly preferred embodiment, both tetrahydroindenyls areunsubstituted.

R″ is preferably an ethylene bridge which is substituted orunsubstituted.

The metal M is preferably zirconium, hafnium or titanium, mostpreferably zirconium. Each Q is the same or different and may be ahydrocarbyl or hydrocarboxy radical having 1-20 carbon atoms or ahalogen. Suitable hydrocarbyls include aryl, alkyl, alkenyl, alkylarylor aryl alkyl. Each Q is preferably halogen. Ethylenebis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride is a particularlypreferred bis tetrahydroindenyl compound of the present invention.

The metallocene catalyst component used in the present invention can beprepared by any known method. A preferred preparation method isdescribed in J. Organomet. Chem. 288, 63-67 (1985).

The cocatalyst which activates the metallocene catalyst component can beany cocatalyst known for this purpose such as an aluminium-containingcocatalyst or a boron-containing codatalyst. The aluminium-containingcocatalyst may comprise an alumoxane, an alkyl aluminium and/or a Lewisacid.

The alumoxanes used in the process of the present invention are wellknown and preferably comprise oligomeric linear and/or cyclic alkylalumoxanes represented by the formula:

for oligomeric, linear alumoxanes and

for oligomeric, cyclic alumoxane,wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R isa C₁-C₈ alkyl group and preferably methyl.

Generally, in the preparation of alumoxanes from, for example, aluminiumtrimethyl and water, a mixture of linear and cyclic compounds isobtained.

Suitable boron-containing cocatalysts may comprise a triphenylcarbeniumboronate such as tetrakis-pentafluorophenyl-borato-triphenylcarbenium asdescribed in EP-A-0427696, or those of the general formula [L′−H]+[B Ar₁Ar₂ X₃ X₄]— as described in EP-A-0277004 (page 6, line 30 to page 7,line 7).

Preferably, the same catalyst system is used in both steps of thecascade polymerisation process to produce a chemical blend of the highand low molecular weight fractions. The catalyst system may be employedin a solution polymerisation process, which is homogeneous, or a slurryprocess, which is heterogeneous. In a solution process, typical solventsinclude hydrocarbons with 4 to 7 carbon atoms such as heptane, tolueneor cyclohexane. In a slurry process it is preferably to immobilise thecatalyst system on an inert support, particularly a porous solid supportsuch as talc, inorganic oxides and resinous support materials such aspolyolefin. Preferably, the support material is an inorganic oxide inits finally divided form.

Suitable inorganic oxide materials which are desirably employed inaccordance with this invention include Group 2a, 3a, 4a or 4b metaloxides such as silica, alumina and mixtures thereof. Other inorganicoxides that may be employed either alone or in combination with thesilica, or alumina are magnesia, titania, zirconia, and the like. Othersuitable support materials, however, can be employed, for example,finely divided functionalised polyolefins such as finely dividedpolyethylene.

Preferably, the support is a silica having a surface area comprisedbetween 100 and 1000 m2/g and a pore volume comprised between 0.5 and 4ml/g.

The amount of alumoxane and metallocenes usefully employed in thepreparation of the solid support catalyst can vary over a wide range.Preferably the aluminium to transition metal mole ratio is in the rangebetween 1:1 and 100:1, preferably in the range 5:1 and 70:1.

The order of addition of the metallocene and alumoxane to the supportmaterial can vary. In accordance with a preferred embodiment of thepresent invention alumoxane dissolved in a suitable inert hydrocarbonsolvent is added to the support material slurried in the same or othersuitable hydrocarbon liquid and thereafter a mixture of the metallocenecatalyst component is added to the slurry.

Preferred solvents include mineral oils and the various hydrocarbonswhich are liquid at reaction temperature and which do not react with theindividual ingredients. Illustrative examples of the useful solventsinclude the alkanes such as pentane, iso-pentane, hexane, heptane,octane and nonane; cycloalkanes such as cyclopentane and cyclohexane;and aromatics such as benzene, toluene, ethylbenzene and diethylbenzene.

Preferably the support material is slurried in toluene and themetallocene and alumoxane are dissolved in toluene prior to addition tothe support material.

In one arrangement according to the present invention, each polyolefinis produced individually in a reactor, preferably a loop reactor, andmixed together by extrusion. The polyolefins may be mixed together bymelt blending. In this way, the low molecular weight and high molecularweight parts of the polyolefin can be produced in separate reactors.

In a preferred arrangement, the product of a first cascade reactionzone, including the olefin monomer, is contacted with the secondco-reactant and the catalyst system in a second cascade reaction zone toproduce and mix the second polyolefin with the first polyolefin in thesecond reaction zone. The first and second reaction zones areconveniently interconnected reactors such as interconnected loopreactors or interconnected loop and continuously stirred reactors. It isalso possible to introduce into the second reaction zone fresh olefinmonomer as well as the product of the first reaction zone.

Because the second polyolefin is produced in the presence of the firstpolyolefin a multimodal or at least bimodal molecular weightdistribution is obtained.

In one embodiment of the invention, the first co-reactant is hydrogenand the second co-reactant is the comonomer. Typical comonomers includehexene, butene, octene or methlylpentene, preferably hexene.

In an alternative embodiment, the first co-reactant is the comonomer,preferably hexene. Because the metallocene catalyst components of thepresent invention exhibit good comonomer response as well as goodhydrogen response, substantially all of the comonomer is consumed in thefirst reaction zone in this embodiment. Homopolymerisation takes placein the second reaction zone with little or no interference from thecomonomer.

In another embodiment, hydrogen may be introduced in both the first andthe second reactor.

The temperature of each reactor may be in the range of from 60° C. to110° C., preferably from 70° C. to 90° C.

The invention will now be described in further detail with reference tothe following non-limiting Examples.

EXAMPLE 1

A. Preparation of the Catalyst

The catalyst comprises ethylene bis(4,5,6,7-tetrahydro-1-indenyl)zirconium chloride metallocene catalyst supported on a silica supportwhich had been treated with a cocatalyst comprising methylalumoxane(MAO).

Silica having a total pore volume of 1.56 ml/g and a surface area of 311m²/g was dried in a fluidised bed-reactor for a period of 6 hours at atemperature of 150° C. under a nitrogen flow of 75 Nl/h. An amount of 10g of the dried silica suspended in 100 ml of dry toluene at roomtemperature was introduced via a dropping funnel into a 500 ml roundbottom flask equipped with a magnetic stirrer, and maintained undernitrogen gas. Thereafter, 40 ml of a 30 wt % solution of methylalumoxane(MAO) in toluene (thereby providing a 1.1 weight ratio between MAO andthe silica) were slowly added to the silica suspension. The solution ofMAO in toluene is commercialy available from the company Witco. Theresulting slurry was heated at a temperature of 110° C. for a period of4 hours. The solid reaction product was then isolated by filtration,using a frit funnel of porosity value 3, and then washed three timeswith 100 ml of dried toluene and then three times with 100 ml of drypentane. The catalytic support was then finally dried under vacuum toproduce 21 g of a free flowing white powder.

Thereafter, for the deposition of the metallocene catalyst onto thesupport as described above, 19 g of the catalytic support was suspendedin 200 ml of dry toluene in a 500 ml round bottom flask equipped with amagnetic stirrer and maintained under nitrogen. Then 1.21 g of ethylenebis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride, alsocommercially available from the company Witco, were added to thesuspension. The amount of the metallocene compound was selected so as toprovide 6 wt % of the metallocene catalyst on the support, based on theweight of the support. The resulting yellow slurry was allowed to reactfor a period of 2 hours. The supported catalyst was then filtered offand washed with successive portions of 100 ml of dry toluene until acolourless filtrate was obtained. The catalyst was then washed threetimes with 100 ml of dry pentane and dried under vacuum. The resultantsupported catalyst was obtained as 20.2 g of a pale yellow free flowingpowder.

B. Preparation of the Low Molecular Weight (LMW) Polyethylene Fraction

Under a flushing of dry nitrogen gas 0.9 millimole of tri-isobutylaluminium (TIBAL) and 900 ml of isobutane were introduced into a dryautoclave reactor having a volume of 3 litres and provided with anagitator. The temperature was raised to 80° C. and hydrogen gas wasadded. Ethylene gas was then introduced until a partial pressure ofethylene of 10×10⁵ Pa was achieved. The amount of hydrogen previouslyintroduced into the autoclave reactor was selected so as to obtain afinal gas phase molar ratio of hydrogen to ethylene (H₂/C₂ molar ratio)of 0.0029 mol/mol.

The polymerisation was then started by flushing into the autoclave thesolid catalyst, comprising 6 wt % of the supported ethylenebis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride metallocenecatalyst (the THI catalyst) prepared hereinabove, in 100 ml ofisobutane. The temperature, partial pressure of ethylene, and the H₂/C₂ratio were kept constant over the polymerisation period, which was 1hour. The reaction was stopped by cooling down, and then venting, thereactor.

The low molecular weight polyethylene was then collected from thereactor. The properties of the low molecular weight polyethyleneobtained are summarised in Table I.

C. Preparation of the High Molecular Weight (HMW) Polyethylene Fraction

The process for preparing the high molecular weight fraction was thesame as that for preparing the low molecular weight fraction specifiedabove in step B, except that instead of adding hydrogen after raisingthe temperature to 80° C., 10 g of 1-hexene comonomer were added and adifferent amount of ethylene was introduced, in order to obtain apartial pressure of ethylene of 8×10⁵ Pa.

The high molecular weight ethylene-hexene copolymer obtained wascollected from the reactor. The properties of the high molecular weightpolyethylene fraction are summarised in Table I.

D. Preparation of the Polyethylene Resin Blend

In order to prepare the blend of the low molecular weight and highmolecular weight polyethylene fractions, 550 g of the low molecularweight polyethylene fraction obtained in step B were then blended with450 g of the high molecular weight ethylene-hexene copolymer obtained instep C. The resulting blend was pelletised twice in an extruder,available in commerce from the company APV Baker under the trade nameMP19TC25.

The properties of the blended polyethylene resin thereby obtained aresummarised in Tables I and II.

EXAMPLE 2

In Example 2, the same metallocene catalyst as in Example 1 was employedto produce both low molecular weight and high molecular weightpolyethylene resin fractions. However, the polymerisation conditions, inparticular the molar ratio of hydrogen to ethylene (H₂/C₂), the 1-hexenecontent and the ethylene partial pressure in steps B and C were variedin order to obtain ethylene polymers having different densities, and inaddition the proportion of the low molecular weight and high molecularweight polyethylene fractions used for making the polyethylene resinblend were changed. The composition and properties of the polyethyleneresins obtained are summarised in Tables I and II.

COMPARATVE EXAMPLES 1 TO 15

In these Comparative Examples, Example 1 was again repeated except thatthe polymerisation conditions, namely the molar ratio of hydrogen toethylene (H₂/C₂), the 1-hexene content and the ethylene partial pressurein steps B and C were adapted in order to obtain ethylene polymershaving different densities and/or melt indices and/or except that theproportions of the low molecular weight and high molecular weightpolyethylene fractions used for making the polyethylene resin blend werechanged. The composition and properties of the polyethylene resinsthereby obtained are summarised also in Tables I and II.

EXAMPLE 3

A. Preparation of Metallocene Catalyst

The metallocene catalyst was the same as that employed in Example 1.

B. Preparation of the Low Molecular Weight (LMW) Polyethylene Fraction

An amount of 0.9 millimole of TIBAL and 900 ml of isobutane wereintroduced under a flushing of dry nitrogen into a dry autoclave reactorhaving a volume of 3 litres and provided with an agitator. Thetemperature was raised to 80° C. and hydrogen gas was added in order toobtain a final hydrogen/ethylene (H₂/C₂) molar ratio of 0.0058 mol/mol.Ethylene was then introduced until a partial pressure of ethylene of10×10⁵ Pa was obtained.

The polymerisation was started by flushing into the autoclave reactorthe catalytic solid which was the same as that employed in Example 1namely comprising 6 wt % of ethylene bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride on the MAO treated silica support in 100 ml ofisobutane. The temperature, partial pressure of ethylene, andhydrogen/ethylene ratio was kept constant during the polymerisationperiod in order to obtain the desired amount of homopolymer. The reactorwas then cooled down and vented until a total pressure of 0.5×10⁵ Pa wasreached. A very small sample of the low molecular weight polyethylenefraction was taken from the reactor in order to measure the density andmelt index (MI₂) of the fraction. The reminder of the low molecularweight polyethylene fraction was left in the reactor.

C. Preparation of the High Molecular Weight (HMW) Polyethylene Fractionin the Presence of the Low Molecular Weight Fraction obtained in Step B.

The temperature of the reactor containing the low molecular weightpolyethylene fraction was then raised up again to 80° C. and 10 g of1-hexene were added. Ethylene was then introduced again into the reactorin order to obtain the desired partial pressure of ethylene of 10×10⁵Pa. The temperature and partial pressure of ethylene were kept constantduring the polymerisation period in order to obtain the desired amountof copolymer, thereby yielding the desired weight ratio between on theone hand the homopolymer produced in the first polymerisation step andon the other hand the copolymer produced in the second copolymerisationstep. The reaction was then stopped by cooling down, and then venting,the reactor.

The resulting polyethylene resin comprising the low molecular weight andhigh molecular weight fractions chemically blended together, was thencollected from the reactor.

D. Pelletisation of the Polyethylene Resin

The polyethylene resin was pelletised using an extruder the same as thatemployed in Example 1. The properties of the pelletised resin blend aresummarised in Tables III and IV.

EXAMPLES 4 to 6

The operations of Example 3 were repeated, except that thepolymerisation conditions (the molar ratio of hydrogen to ethylene, the1-hexene content and the ethylene partial pressure) in steps B and Cwere varied in order to obtain ethylene polymers having differentdensities and/or melt indices, and except that the proportions of thelow molecular weight and high molecular weight polyethylene fractions inthe blend were changed by varying the duration of the first and secondpolymerisation steps. The composition and properties of the resultantpolyethylene resins thereby obtained are summarised also in Tables IIIand IV.

COMPARATIVE EXAMPLES 16 to 19

In these Comparative Examples, the operations of Example 3 were againrepeated, except that the polymerisation conditions (molar ratio) ofhydrogen to ethylene, 1-hexene content, and ethylene partial pressure inthe first and second polymerisation steps were adapted in order toobtain ethylene polymers having different densities and/or melt indicesand/or except that the proportions of the low molecular weight and highmolecular weight polyethylene fractions of the blend were changed byadapting the duration of the first and second polymerisation steps.

The composition and properties of the resultant polyethylene resins aresummarised in Tables III and IV.

EXAMPLE 7

A. Catalyst Preparation

Ethylene bis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride wasprepared in accordance with the method of Brintzinger as published inJournal of Organometallic Chemistry, 288 (1985) pages 63 to 67.

The support used in a silica having a total pore volume of 4.217 ml/gand a surface area of 322 m²/g. This silica was further prepared bydrying in high vacuum on a schlenk line for three hours to remove thephysically absorbed water. 5 g of this silica were suspended in 50 ml oftoluene and placed in a round bottom flask equipped with magneticstirrer, nitrogen inlet and dropping funnel.

An amount of 0.31 g of the metallocene was reacted with 25 ml ofmethylalumoxane (MAO 30 wt % in toluene) at a temperature of 25° C. fora period of 10 minutes to give a solution mixture of the correspondingmetallocenium cation and the anionic methylalumoxane oligomer.

Then the resulting solution comprising the metallocenium cation and theanionic methylalumoxane oligomer was added to the support under anitrogen atmosphere via the dropping funnel which was replacedimmediately after with a reflux condenser. The mixture was heated to110° C. for 90 minutes. Then the reaction mixture was cooled down toroom temperature, filtered under nitrogen and washed with toluene.

The catalyst obtained was then washed with pentane and dried under amild vacuum.

B. Preparation of the Polyethylene Resin

The manufacture of a polyethylene resin comprising a blend of lowmolecular weight and high molecular weight ethylene polymers was carriedout in suspension in isobutane in two loop reactors connected in series.

Isobutane, ethylene, hydrogen, triisobutyl aluminium (TIBAL) and thecatalyst (prepared according to the procedure described in section Aabove) were continuously introduced into the first loop reactor and thepolymerisation of ethylene was carried out in this mixture in order toform the low molecular weight homopolymer. The polymerisation conditionsare specified in Table V. The mixture, additionally comprising the lowmolecular weight homopolymer, was continuously withdrawn from the firstloop reactor and was subjected to a reduction in pressure, so as toremove the hydrogen. The resulting mixture was then continuouslyintroduced into a second loop reactor serially connected to the firstloop reactor together with ethylene, 1-hexene and isobutane. Thepolymerisation of the ethylene and 1-hexene was carried out therein inorder to form the high molecular weight copolymer. The suspensioncomprising the polyethylene resin blend of the low molecular weight andhigh molecular weight ethylene polymer fractions was continuouslywithdrawn from the second loop reactor. The suspension was subjected toa final reduction in pressure, so as to evaporate the isobutane and theremaining reactants present (ethylene, 1-hexene and hydrogen) and torecover the polyethylene resin in the form of a powder, which wassubject to drying in order to complete the de-gasing of the isobutane.The polymerisation conditions in the second reactor are also specifiedin Table V.

The properties of both the low molecular weight and high molecularweight polyethylene resin fractions and also of the ultimatepolyethylene resin, are specified in Table VI, together with furthermeasurements of the capillary viscosity and dynamic viscosity of theultimate polyethylene resin blend.

C. Use of the Composition for the Preparation of Pipes

The polyethylene resin obtained in step B was mixed with 3000 ppm byweight of an anti-oxidising agent, which comprised the compound havingthe trade name Irganox B225 available in commerce from the company CIBASpeciality Chemicals and with 2.25% by weight of carbon black. Theresultant compound was granulated by extrusion in a twin screw extruderavailable under the trade name ZSK 58 from the company Werner &Pfleiderer at a temperature of 215° C.

Thereafter, pipes were manufactured by extrusion of these granulesthrough a single screw extruder at a temperature of 200° C. Pipes havinga diameter of 110 mm were extruded through a single screw extruderavailable in commerce from the company Krauss Mafei under the trade nameKME 1-70-30B. These pipes had an SDR value of 11, the SDR being theratio of external diameter to thickness. Also, 32 mm pipes were producedon a Reifenhauser extruder.

The resins were tested to determined the environmental stress crackresistance (ESCR) using the FNCT and notched pipe tests, the creepresistance, the rapid crack propagation and the Charpy impact energy.The results are summarised in Table VII.

EXAMPLE 8

Example 7 was repeated by using different polymerisation conditions inboth the first and second reactors and the different conditions aresummarised in Table V. The resultant properties of the low molecularweight and high molecular weight fractions and also of the ultimateresin are also summarised in Table VI. Table VI also specifies thecapillary and dynamic viscosity of the ultimate resin. The resin wasalso subjected to the same ESCR (using the FNCT and notched pipe tests),creep resistance, rapid crack propagation and Charpy impact energy testsas the resin of Example 7 and the results are summarised in Table VII.

COMPARATIVE EXAMPLE 20

The polyethylene resin of this Comparative Example comprised acommercially available PE100 compound and the properties of the compoundare summarised in Tables VI and VII.

COMPARISON OF RESULTS

From a comparison of Examples 1 and 2 with Comparative Examples 1 to 15it may be seen that the resin in accordance with the invention has asignificantly higher environmental stress cracking resistance asmeasured by the Bell test than the resin blends not produced inaccordance with the invention. Furthermore, from a comparison ofExamples 3 to 6 with the results of Comparative Examples 16 to 19, itmay also be seen that the environmental stress cracking resistance asmeasured both by the Bell test and by the FNCT test is significantlyenhanced with the resin blends produced in accordance with the presentinvention. Furthermore, when the resin of Examples 7 and 8 is comparedto the resin of Comparative Example 20, corresponding to a commerciallyavailable PE100 compound, it may be seen that the resin produced inaccordance with the invention has good environmental stress crackresistance as determined both by the FNCT test and the notch pipe test,and significantly higher creep resistance than that of the ComparativeExample. In addition, the resin produced in accordance with theinvention has resistance to rapid crack propagation and a Charpy impactenergy which substantially correspond to or are greater than those ofthe known commercially available PE100 resin.

Furthermore, it may be seen that the polyethylene resins of theinvention have a capillary viscosity μ₂ lower than that of commercialPE100 resins which are Ziegler-Natta pipe resins. Accordingly, at highershear rates the resins of the invention can exhibit improvedprocessability as compared to the known PE100 resins. In addition, thepolyethylene resins produced in accordance with the invention also havean η_(0.01) significantly greater than 200,000 Pa·s, the typical maximumvalue for commercially available PE100 Ziegler-Natta pipe resins.Accordingly at lower shear rates the resins of the invention can exhibitimproved resistance to sagging for extruded pipes, for example, ascompared to known PE100 resins. In addition, the resins produced inaccordance with the invention, in general have an η_(0.01)/η₁ greaterthan 8, most often greater than 10, which is greater than the maximumvalue of 8 typically exhibited by commercially available PE100Ziegler-Natta pipe resins. This again demonstrates that the resins ofthe invention exhibit higher viscosity at low shear rates and lowerviscosity at high shear rates as compared to known PE100 resins. Thismeans that the resins of the invention exhibit in combination not onlyimproved processability, particularly for extrusion of pipes andfittings, but also improved resistance to sagging following pipeextrusion. TABLE I LMW FRACTION HMW FRACTION RESIN BLEND p1 MI2 Densityp2 HLMI Density MI5 HLMI Density 1-Hexene ESCR Bell wt % g/10′ Kg/m³Mw/Mn wt % g/10′ Kg/m³ Mw/Mn dg/min dg/min kg/m3 g/Kg Mw/Mn h Ex. 1 0.55185.0 972.5 2.6 0.45 0.24 926.0 3.7 0.63 22.0 952.5 5.0 10.5 >850 Ex. 20.54 200.0 972.7 2.5 0.46 <0.1 924.0 4.6 0.28 13.0 951.4 7.0 11.7 >500Comp. Ex 1 0.58 103.0 970.8 0.42 0.19 931.5 4.7 0.84 28.0 956.9 3.0 11<18 Comp. Ex 2 0.65 11.5 964.7 2.8 0.35 0.17 933.6 4.6 0.83 17.0 955.07.2 <18 Comp. Ex 3 0.75 3.3 961.1 2.9 0.25 0.30 934.8 4.7 1.50 22.0955.1 4.5 <18 Comp. Ex 4 0.85 2.1 959.9 2.9 0.15 0.10 927.8 5.6 1.7023.0 955.2 3.0 4.4 <18 Comp. Ex 5 0.80 2.1 959.9 2.9 0.20 0.16 927.8 4.41.00 17.0 949.1 9.0 4.6 <18 Comp. Ex 6 0.64 21.0 966.4 2.7 0.36 0.10927.8 5.6 0.78 19.4 953.5 3.0 7.6 <18 Comp. Ex 7 0.50 185.0 972.5 2.60.50 0.39 935.2 4.6 0.52 15.9 956.2 3.0 10.9 <23 Comp. Ex 8 0.52 200.0972.7 2.5 0.48 0.17 933.6 4.6 0.36 13.6 956.8 4.0 12.6 <23 Comp. Ex 90.56 45.0 968.5 2.7 0.44 0.17 933.6 4.6 0.48 13.3 954.3 9.2 <17 Comp. Ex10 0.59 32.0 967.5 3.1 0.41 0.10 927.1 4.3 0.79 21.0 954.5 2.0 9.3 20Comp. Ex 11 0.66 11.5 964.7 2.8 0.34 <0.1 923.5 0.82 17.5 950.3 7.0 6.450 Comp. Ex 12 0.49 200.0 972.7 2.5 0.51 0.17 933.6 4.6 0.23 8.3 955.211.7 <15 Comp. Ex 13 0.52 13.0 965.0 3.1 0.48 0.17 933.6 4.6 0.24 6.1952.4 8.3 15-24 Comp. Ex 14 0.70 0.6 956.3 3.4 0.30 0.39 935.2 4.6 0.396.5 950.7 4 <17 Comp. Ex 15 0.84 0.4 954.8 3.3 0.16 0.22 920.0 4.0 0.628.5 950.2 4.0 3.8 <15

TABLE II μ2 η0.01 η1 dPa · s Pa · s Pa · s η0.01/η1 Ex. 1 17300 26000025136 10.3 Ex. 2 19000 404630 35829 11.3 Comp. Ex. 1 16200 193190 209809.2 Comp. Ex. 2 21900 121160 19687 6.2 Comp. Ex. 3 20800 70359 13901 5.1Comp. Ex. 4 20600 51105 12096 4.2 Comp. Ex. 5 23300 108610 17774 6.1Comp. Ex. 6 19900 136230 19823 6.9 Comp. Ex. 7 19400 286100 28271 10.1Comp. Ex. 8 19500 346120 33574 10.3 Comp. Ex. 9 21100 225350 28229 8.0Comp. Ex. 10 18700 170350 22582 7.5 Comp. Ex. 11 21000 137710 20286 6.8Comp. Ex. 12 22100 491710 42100 11.7 Comp. Ex. 13 28900 412620 3911810.5 Comp. Ex. 14 35400 265750 28421 9.4 Comp. Ex. 15 32900 204300 259817.9

TABLE III LMW Fraction HMW Fraction (1st Step) (2nd Step) RESIN BLENDESCR p1 MI2 Density p2 HLMI* Density* MI5 HLMI Density Hexene Bell TestFNCT wt % g/10′ Kg/m³ wt % g/10′ Kg/m³ dg/min dg/min kg/m³ g/Kg Mw/Mn hh EX. 3 0.58 330.0 973.0 0.42 0.04 922.4 0.41 18.5 953.3 4 14.3 >500 627EX. 4 0.60 335.0 973.3 0.4 0.02 917.7 0.38 19.9 952.5 6 14.9 >500 >4820EX. 5 0.54 134.0 971.0 0.46 0.02 921.3 0.19 7.1 949.8 7 11.1 >400 1445Ex. 6 0.60 251.0 973.0 0.4 0.05 916.4 0.62 24.2 951.8 9 13.0 >500 2479Comp. Ex. 16 0.90 0.99 957.4 0.10 0.02 913.8 1.17 15.9 953.4 2 4.8 <16Comp. Ex. 17 0.80 1.2 956.9 0.20 0.27 927.2 0.90 15.2 951.7 4.5 4.720-24 Comp. Ex. 18 0.60 66.6 968.9 0.40 0.02 939.2 0.28 11.4 958.5 10.4<16 Comp. Ex. 19 0.53 160.0 972.1 0.47 0.02 934.1 0.14 5.7 956.0 12.516.25 6*calculated

TABLE IV μ2 η0.01 η1 dPa · s Pa · s Pa · s η0.01/η1 Ex. 3 16900 45509033241 13.7 Ex. 4 16200 525030 35742 14.7 Ex. 5 23100 734060 44582 16.5Ex. 6 15800 348480 27667 12.6 Comp. Ex. 16 23700 103370 16586 6.2 Comp.Ex. 17 23000 179440 22649 7.9 Comp. Ex. 18 20100 553330 38758 14.3 Comp.Ex. 19 23800 825220 51787 15.9

TABLE V Example 7 Example 8 REACTOR 1 C₂ (% mol) 20.9 16.2 comonomer — —H₂/C₂ (% mol/mol) 0.0588 0.0427 T (° C.) 80 80 residence time (h) 1.821.83 REACTOR 2 C₂ (% mol) 12.80 14.97 C₆/C₂ (% mol/mol) 3.1 9.1 H₂/C₂ (%mol/mol) — — T (° C.) 75 75 residence time (h) 0.89 0.87

TABLE VI Comp. Example 20 Example Example (Commercial PE 7 8 100) LMWfraction (reactor 1) weight (%) 56.9 55.5 — MI₂ (g/10 min) 459 149 —Density (kg/m³) 974.2 971.0 — HMW fraction (reactor 2) weight (%) 43.144.5 — HLMI (g/10 min)* 0.03 0.04 — Density (kg/m³)* 919.4 919.6 —Polyethylene resin blend hexene g/kg 9 10 — MI₅ (g/10 min) 0.26 0.2 0.34 HLMI (g/10 min) 15.6 10.1 10.7 Density (kg/m³) 952.2 949.5 ˜950** μ₂ (dPa · s) 16,800 19,900 >21,000    η_(0.01) (Pa · s) 587,960 675,480≦200,000     η₁ (Pa · s) 39,722 43,426 — η_(0.01)/η₁ 14.8 15.6 <8 *calculated**commercial PE 100 resins usually have a density around 960 kg/m³, butcontain carbon black; the density of the resin can therefore beestimated as around 950 kg/m³.

TABLE VII Comparative Example 20 (Commercial PE Example 7 Example 8 100)ESCR FNCT >4700 >4700 300 5 MPa, 80° C. (h) Notched pipe test 3081 >21821000 4.6 MPa, 80° C. (h) Creep Resistance 20° C. (h) at 13.0 MPa >2000400 200 Rapid Crack −12.5 to −15° C. <−20° C. −5 to −15° C. PropagationCritical temperature (critical pressure 10 bars) Charpy Impact Energy 1419 10 (kJ/m²) (−25° C.)

1-15. (canceled)
 16. A process for the preparation of a polyethyleneresin having a bimodal molecular weight distribution which comprises:(a) contacting ethylene monomer and a first co-reactant selected fromthe group consisting of hydrogen and a comonomer comprising a C₃-C₁₂alpha olefin in a first reaction zone under first polymerizationconditions to produce a first polyethylene in the presence of a firstcatalyst system which includes a metallocene catalyst componentcomprising a bridged bis(tetrahydroindenyl) compound of the generalformula R″(IndH₄)₂MQ₂ in which each IndH₄ is the same or different andis tetrahydroindenyl or substituted tetrahydroindenyl, R″ is a bridgebetween the IndH₄ groups which comprises a C₁-C₄ alkylene radical, adialkyl germanium or silicon or siloxane, or an alkyl phosphine or amineradical, which bridge is substituted or unsubstituted, M is a Group IVmetal or vanadium and each Q is a hydrocarbyl having 1 to 20 carbonatoms or a halogen; and a cocatalyst which activates the catalystcomponent; (b) contacting ethylene monomer and a second co-reactantselected from the group consisting of hydrogen and a comonomercomprising a C₃-C₁₂ alpha olefin in a second reaction zone under secondpolymerization conditions to produce a second polyethylene differentfrom said first polyethylene in the presence of a second catalyst systemwhich includes a metallocene catalyst component comprising a bridgedbis(tetrahydroindenyl) compound of the general formula R″(IndH₄)₂MQ₂ inwhich each IndH₄ is the same or different and is tetrahydroindenyl orsubstituted tetrahydroindenyl, R″ is a bridge between the IndH₄ groupswhich comprises a C₁-C₄ alkylene radical, a dialkyl germanium or siliconor siloxane, or an alkyl phosphine or amine radical, which bridge issubstituted or unsubstituted, M is a Group IV metal or vanadium and eachQ is hydrocarbyl having 1 to 20 carbon atoms or halogen; and acocatalyst which activates the catalyst component, and (c) blendingtogether said first and second polyethylenes to form a polyethyleneresin comprising a blend of from 35 to 49 wt. % of a polyethylenefraction of high molecular weight and from 51 to 65 wt. % of a lowmolecular weight polyethylene fraction having a lower molecular weightthan said high molecular weight polyethylene fraction, said highmolecular weight polyethylene fraction comprising a linear low densitypolyethylene having a density of no more than 0.930 g/cm³, and a highload melt index, HLMI, of less than 0.6 g/10 min. and the lowermolecular polyethylene fraction comprising a high density polyethylenehaving a density of at least 0.969 g/cm³ and a melt index, MI₂, ofgreater than 10 g/10 min., said polyethylene resin having a densitygreater than 0.946 g/cm³, a high load melt index, HMLI, within the rangeof 1-100 g/10 min., a dynamic viscosity η_(0.01) measured at 0.01radian/second, which is greater than 200,000 Pa·s, and a ratioη_(0.01)/η₁ which is greater than 8; wherein (i) η_(0.01) is the dynamicviscosity measured at 0.01 radian/second; and (ii) η₁ is the dynamicviscosity of the polyethylene resin measured at 1 radian/second.
 17. Theprocess of claim 16 wherein said first polyethylene comprises a highdensity ethylene homopolymer, and said second polyethylene comprises acopolymer of ethylene and a C₃-C₁₂ alpha olefin comonomer.
 18. Theprocess of claim 17 wherein said C₃-C₁₂ alpha olefin in said secondpolymer comonomer is selected from the group consisting of butene,hexane, hexene, methylpentene, octene and mixtures thereof.
 19. Theprocess of claim 17 wherein said comonomer in said second polymercomprises hexene.
 20. The process of claim 17 wherein said first andsecond reaction zones are connected in series and said firstpolyethylene is supplied from said first reaction zone to said secondreaction zone.
 21. The process of claim 20 wherein said firstco-reactant is hydrogen and said second co-reactant comprises a C₃-C₁₂alpha olefin.
 22. The process of claim 20 wherein said first co-reactantcomprises a C₃-C₁₂ alpha olefin as a comonomer, wherein said firstethylene polymer is a copolymer of ethylene and said comonomer which issupplied from said first reaction zone to said second reaction zone andthe second co-reactant in said second reaction zone is hydrogen, whereinhomopolymerization takes place in said second reaction zone.
 23. Theprocess of claim 16 wherein said first and second polyethylenes areblended together to form said polyethylene resin comprising a blend ofno more than 45 wt. % of said high molecular weight polyethylenefraction and at least 55 wt. % of said low molecular weight fraction.24. The process of claim 16 wherein said first and second polyethylenesare blended together to form a polyethylene resin having a ratioη_(0.01)-η₁ which is greater than
 10. 25. The process of claim 16wherein said first and second polyethylenes are blended together to formsaid polyethylene resin comprising a blend of no more than 44 wt. % ofsaid high molecular weight polyethylene fraction and at least 56 wt. %of said low molecular weight fraction.
 26. The process of claim 16wherein said polyethylene resin has a density of at least 0.954 g/cm³.27. The process of claim 16 wherein R″ in each of the metallocenecatalyst components of subparagraphs (a) and (b) is an ethylene bridge.